Semiology and semiotics are two related disciplines, which study semiosis, the relation of signification involving sign, object and mind, and classification of signs. Morris has classified three dimensions of semiosis: 1) the syntactic dimensions i.e. relations between signs, 2) semantic dimension i.e, relations between signs and objects, and 3) pragmatic dimension, i.e. relations between the sign and interpreter. Shepperson and Tomaselli have tried to describe the differences between semiology and semiotics in relation to European vs. African culture. In the following we try to introduce the basic ideas of semiology and semiotics. See also Lehm's "semionomy".
Semiology has its basis in Kantian dichotomy of phenomenal (mental) and noumenal (material) worlds, which corresponds the classic European dichotomy of subjective and objective. De Saussure (1857-1913) founded the idea of semiology as the science of signs. Sign is a conceptual object, which consists of signifier (the name of sign) and signified (the referred idea in the mind, concept or meaning). In addition there are perceptual objects or referents (the real objects), but the signs do not refer to them, but only the concepts in our mind. The goal of semiology is to determine the relations between the signifier and signified in the given language context. De Saussure argues that the names (signifiers) and their relations to signified ideas are pure arbitrary, and there doesn't exist any fixed universal ideas, but they are also arbitrary and depend on language. Shepperson and Tomaselli remind that the semiology can easily lead to a solipsistic view: semiology is itself just a linguistic structure, and we are caught by it without any reference to real world.
Peirce (see also constructing beliefs) rejected the dualistic ontology behind the semiology and constucted a triadic view of world, which is represented in semiotics. He studied the triadic relation between the sign, the object and the mind. He argued that we cannot fully reach the material reality by our experiences. The signs construct the relation between the mind and experience, and they signify completely when they cause a habit change in the interpreter (we could call this "deep learning"). The most effective change in habit can also produce new signs or new uses of signs. So the signs have meaning only in relation to mind and habits. (Question: what did Peirce think about relation between the sign and the real objects? Does it exist at all? Or does it exist only mediatively, by sign-mind and object-mind relations?)
The triadic nature concerns also understanding. If the sign means something, it requires somebody (a mind) signifying and something (an object) that is signified. In addition, the signs themselves have triadic nature, and Peirce lists several triads. A trichotomy of icon, index and symbol deals with the way signs are recognized. Another trichotomy of qualisign, sinsign and legisign classify the signs according to the kind of act of signification that is taking place. Qualisign, sinsign and legisign also correspond the levels of comprehension: qualisign - the surroundings of the subject (phaneron), sinsign - that which can be separated from overall context, and legisign - the relations between what has been separated out. (I must confess that all this is very obscure and I didn't understand it completely!)